Recently, surface scattering antennas and other such radio-frequency devices have been disclosed that use a liquid crystal (LC)-based metamaterial antenna element as part of the device. In the case of antennas, the LCs have been used as part of the antenna elements for tuning the antenna element. For example, LC is placed between two glass substrates that comprise an antenna array using liquid crystal display (LCD) manufacturing processes well-known in the art of LCDs. These glass substrates are spaced apart using gap spacers and are sealed at the edge using some type of sealant (e.g., adhesive).
The volume of an empty liquid crystal cell over a temperature range is controlled by the coefficient of thermal expansion (CTE) of the glass substrates, gap spacers and the edge seal. The volume change of liquid crystal due to temperature change in a liquid crystal cell will be greater than the cavity volume change of the LC cell itself, because the volume expansion coefficient of the LC is much larger than the CTE of the LC cell components.
As temperatures rise, the total change in volume of the LC will be greater than the cavity volume increase, and the liquid crystal gap will no longer be controlled by the seal and spacers, leading to a greater-than-desired cell gap, a decrease in LC gap uniformity, and a shift in the resonant frequency of the elements that are affected. This non-uniformity results from the gap no longer being controlled by the spacers. Once there is no longer sufficient pressure on the substrates to hold the substrates on the spacers due to the LC's volume expansion, the gap will be controlled by other mechanical considerations. In other words, the increase in volume will not result in uniform gap distribution, and the LC will move to achieve mechanical equilibrium without control by the spacers. This means that the LC may pool in locations to best relieve the mechanical stresses. For example, the cell gap near the seal area is fixed by the spacers/adhesive. If everything else were perfect, at higher temperatures, the LC thickness distribution over the segment area will show a greater thickness in the center of the aperture than at the edge, because the cell gap near the edges of the cell is controlled by the border seal adhesive, a lower thermal expansion material than the liquid crystal.
As temperatures decrease, the volume of LC will be less than the LC cell cavity volume, reducing the internal pressure of the LC cell. Atmospheric pressure will then push the glass down tighter on the cell spacers, reducing the cell gap if the modulus of elasticity of the spacers is such that the increasing pressure on the spacers can compress the spacers. If the difference in volume is great enough, this can result in places where the LC volume has been replaced by residual gas that was dissolved in the LC. The immediate result of this condition may be voids in places in the aperture where the dielectric of the LC has been replaced with residual gas affecting antenna element performance. Once the cell warms up sufficiently, it may take time for these voids to disappear (if there is sufficient gas in the voids, the gas may need to re-dissolve for the void to disappear). Additionally, in the locations where the voids formed, alignment defects may be present.
A similar problem to the low temperature case can result from being at lower atmospheric pressures, such as those that arise at higher altitudes. In this case, the pressure exerted on the substrates (holding the substrates on their spacers) is reduced. Non-uniformity and voids can result.
Thus, the change in LC cell gap and increase in LC cell gap non-uniformity with ambient temperature and pressure changes are problematic for forming RF antenna elements that function correctly.